Surface acoustic wave filter

ABSTRACT

A surface acoustic wave filter having at least one transmission line of which one end of a plurality of the transmission lines is connected to the surface acoustic wave element and the other end thereof is connected to a ground, and the transmission lines being divided by a transmission line which is connected to a ground electrode at an interval equal to or lower than a wavelength of a using frequency. This structure provides grounds among a plurality of transmission lines and thus isolation among terminals can be increased to improve the filter characteristic.

TECHNICAL FIELD

The present invention relates to a surface acoustic wave filter used forvarious communication devices.

PRIOR ART

A conventional surface acoustic wave filter (hereinafter referred to asSAW filter) will be described with reference to FIG. 14. Theconventional SAW filter has: first dielectric layer 72, seconddielectric layer 78, cavity 80, and metal plate 81. First dielectriclayer 72 has, at the lower face thereof, transmission terminal 68,reception terminal 69, antenna terminal 70, and first ground electrode71 and has, at the upper face thereof, transmission lines 73, 74, 75,76, and 77. Second dielectric layer 78 is provided so as to be opposedto transmission lines 73, 74, 75, 76, and 77 has at the upper facethereof second ground electrode 79. Cavity 80 is provided so as to beopposed to second ground electrode 79 and has at the center thereof anopening section to provide a hollow shape. The upper side of cavity 80is welded with metal plate 81 to close the opening section of cavity 80,thereby providing a package.

Surface acoustic wave element 82 is mounted at the upper face of secondground electrode 79. This surface acoustic wave element 82 is providedso as not to have a contact with cavity 80 and metal plate 81. Withregards to the conventional SAW filter having the structure as describedabove, the passage characteristic between transmission terminal 68 andantenna terminal 70 only with respect to the package (i.e., isolationcharacteristic) except for surface acoustic wave element 82 is shown byline A in FIG. 15. As can be seen from line A, the attenuation at 2.17GHz is 56.8 dB.

Broken line M in FIG. 15 shows the frequency characteristic of theconventional SAW filter as described above when the SAW filter has acircuit as shown in FIG. 6 and has the frequency characteristic as shownin FIG. 7. As can be seen from FIG. 15, the SAW filter has attenuationin the frequency band from 2.11 GHz to 2.17 GHz of 57.8 dB, which islarger than the attenuation of the package (56.8 dB). Thus, largeramount of signal propagates the package between transmission terminal 68and antenna terminal 70

As described above, the conventional package had an insufficientisolation characteristic in order to provide an SAW filter having alarge attenuation.

SUMMARY OF THE INVENTION

The surface acoustic wave filter of the present invention has at leastone transmission line whose one end is connected to a surface acousticwave element and the other end is connected to ground, and thetransmission lines are divided by a transmission line which is connectedto a ground electrode at an interval equal to or lower than a wavelengthof a using frequency. This structure can increase the isolation amongterminals and thus the filter characteristic can be improved.

The surface acoustic wave filter of the present invention also has astructure in which a transmission terminal, a reception terminal and anantenna terminal are divided by a transmission line connected to aground at an interval equal to or lower than a wavelength of a usingfrequency. This structure can increase the isolation among terminals ofthe transmission terminal, the reception terminal and the antennaterminal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view illustrating an SAW filter of oneembodiment of the present invention.

FIG. 2A is a top view illustrating an SAW filter of one embodiment ofthe present invention.

FIG. 2B is a cross-sectional view illustrating an SAW filter of oneembodiment of the present invention.

FIG. 3 is a magnified view illustrating a connection via hole part of anSAW filter of one embodiment of the present invention.

FIG. 4 is a magnified view illustrating a connection via hole part of anSAW filter of one embodiment of the present invention.

FIG. 5 is a magnified view illustrating a connection via hole part of anSAW filter of one embodiment of the present invention.

FIG. 6 is a circuit diagram of an SAW filter of one embodiment of thepresent invention.

FIG. 7 is a frequency characteristic diagram of an SAW filter of oneembodiment of the present invention.

FIG. 8 is a frequency characteristic diagram of an SAW filter packagepart of one embodiment of the present invention.

FIG. 9 is a magnified view illustrating a connection via hole part ofanother embodiment of the present invention.

FIG. 10 is a magnified view illustrating a connection via hole part ofanother embodiment of the present invention.

FIG. 11 is a magnified view illustrating a connection via hole part ofanother embodiment of the present invention.

FIG. 12 is a magnified view illustrating a connection via hole part ofanother embodiment of the present invention.

FIG. 13 is a frequency characteristic diagram of a package part ofanother embodiment of the present invention.

FIG. 14 is an exploded perspective view illustrating a conventionalexample.

FIG. 15 is a frequency characteristic diagram of a package part of aconventional example.

DETAILED DESCRIPTION OF THE INVENTION

In a surface acoustic wave filter of the present invention (hereinafterreferred to as SAW filter), grounds are provided among a plurality oftransmission lines of which one end is connected to a surface acousticwave element and the other end is connected to a ground. As a result,isolation between terminals can be increased to improve the filtercharacteristic.

The SAW filter of the present invention also provides a ladder typefilter in which SAW resonators connected serially and SAW resonatorsconnected in parallel are connected in a ladder-like manner and canincrease the freedom in the design of the SAW filter.

In the SAW filter of the present invention, at least one dielectriclayers have thereon transmission line that can be provided to have amulti-layer structure, thus increasing the freedom in the design of thetransmission line.

Also in the SAW filter of the present invention, the connection via holehas a diameter that is smaller than the width of the transmission lineto be connected. Thus, a land electrode for connecting the transmissionline with a connection via hole is not required and thus the freedom inthe design of the transmission line can be increased. The term“connection via hole” used herein refers to a via hole having an innerside provided with plating or a via hole filled with a conductivematerial that is used for interlayer connection in a laminatedstructure.

Also in the SAW filter of the present invention, the surface acousticwave element is covered by a metal plate so that the surface acousticwave element is sealed in an air-tight manner and thus the filter canhave an increased reliability.

Also in the SAW filter of the present invention, a layer having atransmission line is sandwiched by ground layers and the respectiveground layers are connected by a connection via hole. This structure canstrengthen the ground and thus a further increased isolation can beobtained.

Also in the SAW filter of the present invention, a transmission lineconnected to a ground at an interval equal to or lower than a wavelengthof a using frequency has at least one branch. Thus, a transmission lineconnected to a ground at an interval equal to or lower than a wavelengthof a using frequency can divide three or more regions. Thus, number of apair of terminals for improving the isolation can be increased.

Also in the SAW filter of the present invention, a transmission lineconnected to a ground at an interval equal to or lower than a wavelengthof a using frequency has at least one or more corner sections. Thecorner section preferably has an angle of 45° or 90°. This structureprovides an easy introduction of a screen printing for providing atransmission line and thus the width of the path can be reduced in aneasy manner.

By connecting the above corner section via the connection via hole tothe ground electrode, a part having an angle can be grounded. A parthaving an angle in the transmission line has impedance different fromthat of a straight part, and thus the former and the latter havedifferent coupling conditions. The structure as described above allowsthe part having the angle to have the same coupling conditions as thosefor the straight part.

Also in the SAW filter of the present invention, a transmissionterminal, a reception terminal and an antenna terminal are divided by atransmission line connected to a ground at an interval equal to or lowerthan a wavelength of a using frequency. Thus, isolations among therespective transmission terminal, reception terminal, and antennaterminal can be increased.

Hereinafter, one embodiment of the present invention will bespecifically described with reference to the drawings.

As shown in FIG. 1, an SAW filter in one embodiment has: firstdielectric layer 5, second dielectric layer 12, cavity 14, and metalplate 15. First dielectric layer 5 has, at the lower face, transmissionterminal 1, reception terminal 2, antenna terminal 3, and first groundelectrode 4 and has, at the upper face, transmission lines 6, 7, 8, 9,10, and 11. Second dielectric layer 12 provided to be opposed totransmission lines 6, 7, 8, 9, 10, and 11 has, at the upper face, secondground electrode 13. Cavity 14 is provided so as to be opposed to secondground electrode 13 and has at the center an opening section to providea hollow shape. The upper side of cavity 14 is welded with metal plate81 to close the opening section of cavity 14, thereby providing apackage.

Second ground electrode 13 has, at the upper face thereof, surfaceacoustic wave element 16. Surface acoustic wave element 16 is providedso as not to have a contact with cavity 14 and metal plate 15.

As shown in FIG. 2A, cavity 14 has a structure surrounding surfaceacoustic wave element 16 provided at the center opening section. Surfaceacoustic wave element 16 is sandwiched by electrode pads 17 a to 17 l.As shown in the cross-sectional view of FIG. 2B, two sides of the innerperiphery of cavity 14 has a stepped shape. Electrode pads 17 a, 17 b,17 c, 17 d, 17 e, and 17 f are formed on a step of one side, andelectrode pads 17 g, 17 h, 17 i, 17 j, 17 k, and 17 l are formed on astep of the other side.

FIG. 3 is a magnified view of the neighborhood of first dielectric layer5, second dielectric layer 12, and cavity 14 in FIG. 1. FIG. 4 is amagnified view of the neighborhood of first ground electrode 4 and firstdielectric layer 5 in FIG. 1. In FIG. 3, one end of transmission line 6is connected to electrode pad 17 through connection via hole 18 shown inFIG. 3, and the other end thereof is connected to first ground electrode4 through connection via hole 23 shown in FIG. 4. Similarly, one end oftransmission line 7 is connected to electrode pad 17 d throughconnection via hole 19 and the other end thereof is connected to firstground electrode 4 through connection via hole 24. One end oftransmission line 8 is connected to electrode pad 17 i throughconnection via hole 20, and the other end is connected to first groundelectrode 4 through connection via hole 25. One end of transmission line9 is connected to electrode pad 17 j through connection via hole 21, andthe other end is connected to first ground electrode 4 throughconnection via hole 26. One end of transmission line 10 is connected toelectrode pad 17 a through connection via hole 22 and a transmissionline (not shown) provided in cavity 14 and the one end is also connectedto antenna terminal 3 by 27, and the other end thereof is connected tofirst ground electrode 4 through connection via hole 28. As shown inFIG. 1, connection via hole 22 is a via hole extending from antennaterminal 3 in a vertical direction.

FIG. 5 is a magnified view illustrating the neighborhood of first groundelectrode 4 and first dielectric layer 5 of FIG. 1. The most importantfeature of the above structure is that the upper face of firstdielectric layer 5 has transmission line 11. This transmission line 11has a shape that has one branching point and three end sections. Thethree end sections are connected to first ground electrode 4 throughconnection via holes 29, 30, and 31. Transmission line 11 is alsoconnected, between the end sections, to first ground electrode 4 throughconnection via holes 32, 33, 34, and 35. This allows transmission line11 to be connected to first ground electrode 4 at an interval equal toor lower than the wavelength of a using frequency. Connection via hole33 is connected to the branching point of transmission line 11 andconnection via hole 35 is connected to a point at which transmissionline 11 is bent to have a right angle.

Next, the operation of the SAW filter of this embodiment will bedescribed. A case in which a circuit shown in FIG. 6 will be describedin detail as an example.

The following section will describe the circuit structure. The surfaceacoustic wave element 16 has at the surface thereof input terminal 36.This input terminal 36 is connected with one end of first serialconnected SAW resonator 37 and one end of first parallel connectedresonator 38. The other end of first serial connected SAW resonator 37is connected with one end of second serial connected SAW resonator 39and one end of second parallel connected SAW resonator 40. The other endof second serial connected SAW resonator 39 is connected with outputterminal 41 provided at the surface of surface acoustic wave element 16.

Input terminal 36 is electrically connected to electrode pad 17 e shownin FIG. 2A. Electrode pad 17 e is connected to transmission terminal 1and output terminal 41 is connected to electrode pad 17 a. The other endof first parallel connected SAW resonator 38 is connected to electrodepad 17 c. The other end of second parallel connected SAW resonator 40 isconnected to electrode pad 17 d. First inductor 42 in FIG. 6 correspondsto transmission line 10 of FIG. 1. Second inductor 43 corresponds totransmission line 6 and thus the other end of first parallel connectedSAW resonator 38 is grounded to first ground 4 from electrode pad 17 cthrough connection via hole 23. Third inductor 44 corresponds totransmission line 7 and thus the other end of second parallel connectedSAW resonator 40 is grounded to first ground 4 from electrode pad 17 dthrough connection via hole 24.

The structure as described above allows input terminal 36 and outputterminal 41 have therebetween the serial connection of first serialconnected SAW resonator 37 and second serial connected resonator 39 andthe parallel connection of first parallel connected SAW resonator 37 andsecond parallel connected resonator 39. As a result, the SAW filter ofthis embodiment provides a ladder-type filter in which two SAWresonators having serial connection and two SAW resonators havingparallel connection are connected in a ladder-like manner.

With regards to the SAW filter having the structure as described above,the frequency characteristic between transmission terminal 1 and antennaterminal 3 is shown in FIG. 7. A range from 1.92 GHz to 1.98 GHz shows apassage characteristic having a small loss and both sides thereof show acharacteristic showing an attenuation. The most remarkablecharacteristic is that a frequency band from 2.11 GHz to 2.17 GHz showsa large attenuation. The reason why such a large attenuation can beobtained will be described with reference to FIG. 8.

Line B of FIG. 8 shows the frequency characteristic between transmissionterminal 1 and antenna terminal 3 in the case of only the SAW filterpackage of this embodiment (i.e., isolation characteristic when surfaceacoustic wave element 16 is removed in this embodiment). On the otherhand, line A shows the isolation characteristic of the conventionalpackage shown in FIG. 15.

As described with reference to FIG. 1, transmission line 6 andtransmission line 10 are provided on first dielectric layer 5 and theelectric field coupling between them is maximum when the former and thelatter are provided to have the minimum distance therebetween.Transmission line 11 is provided on dielectric layer 5 so as to separatetransmission line 6 from transmission line 10. Since transmission line11 is connected to first ground electrode 4 through connection via holes29, 30, 31, 32, 33, 34, and 35 as shown in FIG. 5, transmission line 11has a potential that is sufficiently low. Thus, transmission line 11provides an effect for reducing the electric field coupling betweentransmission line 6 and transmission line 10. As a result, isolationbetween transmission terminal 1 and antenna terminal 3 can be increased.

First dielectric layer 5, second dielectric layer 12, and cavity 14 areprepared by low-temperature co-fired ceramics having alumina as a mainelement and having a dielectric constant of 7.8 to provide a packagehaving a shape of 3.8 mm×3.8 mm×1.3 mm. Transmission line 6,transmission line 7, transmission line 10, and transmission line 11 areformed by a conductor including silver as a main element to have a linewidth of 100 μm. Transmission line 6 and transmission line 10 are formedto have the minimum distance therebetween of 200 μm. Transmission line11 is provided at a position at which transmission line 6 andtransmission line 10 have the minimum distance therebetween and isformed so as to separate transmission line 6 from transmission line 10while including the center part located between transmission line 6 andtransmission line 10. Each connection via hole is formed by a conductorincluding silver as a main element to have a diameter of 100 μm. A partof each transmission line at which each connection via hole is connectedis formed to have a line width of 200 μm.

Transmission line 11 is connected through connection via holes 29 to 35to ground electrode 4 at an interval equal to or lower than a wavelengthof 2.17 GHz. This allows the impedance among the connection via holes tobe small when compared to a using frequency and reduces the impedance intransmission line 11 to a negligible level.

When the electric field coupling between transmission terminal 1 andantenna terminal 3 of the package having the structure as describedabove is represented as a capacitor, the electric field coupling isabout 0.00095 pF at 2.17 GHz, which is about 20% lower when compared to0.0012 pF in the conventional example. The frequency characteristic ofthe attenuation is shown by line B of FIG. 8. As can be seen from lineB, the attenuation at 2.17 GHz is 58.8 dB. This value exceeds even 57.8dB that is the maximum attenuation of the SAW filter shown by brokenline M in FIG. 7 and FIG. 8. Therefore, signals propagating fromtransmission terminal 1 via the package to antenna terminal 3 in a bandfrom 2.11 GHz to 2.17 GHz is smaller than signals propagating fromtransmission terminal 1 via surface acoustic wave element 16 to antennaterminal 3. As a result, the package having this structure can providethe SAW filter having the characteristic shown in FIG. 7.

Surface acoustic wave element 16 of this embodiment is preferablyprepared by lithium tantalate because an angle at which the element iscut has an increased freedom to provide a filter having a lower loss inan easy manner. An angle at which surface acoustic wave element 16 ofthis embodiment is cut is 39°.

In the above embodiment, transmission lines 11 at parts in whichtransmission line 11 is connected to connection via holes 29, 30, and 31have an increased line width to provide land electrodes 100.Consequently, the transmission line at the connection part is providedwith a width that is larger than the diameter of the connection viahole. Alternatively, the diameter of the connection via hole may beprovided to be smaller than the line width of transmission line 11. Inthis case, it is not necessary for transmission line 11 to have landelectrode 100 for the connection to connection via holes 29 to 35 andthus transmission line 11 can have an increased freedom in the design.

Although transmission line 11 in the above embodiment is bent at rightangle at one point, transmission line 11 also may be bent at an angle of45°. In the latter case, a screen printing for providing transmissionline 11 can be performed in a manner in which the path width iscontrolled easily and thus the line width can be further reduced and theSAW filter can have a smaller size without having any small influence onthe characteristic even when a fine line needs to be formed.

Although transmission line 11 in the above embodiment is provided onfirst dielectric layer 5, one transmission line may be divided into aplurality of layers. FIG. 9 shows an example in which third dielectriclayer 45 is inserted between first ground electrode 4 and firstdielectric layer 5. FIG. 9 illustrates a structure in which thirddielectric layer 45 has thereon transmission line 46, one end oftransmission line 46 is connected to the other end of transmission line7 through connection via hole 47, and the other end of transmission line46 is connected to first ground electrode 4 through connection via hole48. The structure as described above provides an increased freedom inthe design of a transmission line to be formed. Specifically, thetransmission line can have an increased inductance component that can beused for the design of the filter in an easier manner. When there aretwo or more transmission lines to be divided to have a plurality oflayers as described above, third dielectric layer 45 may have thereontransmission line 49 and transmission line 49 is connected totransmission line 11 and first ground electrode 4 through connection viaholes 50, 51, 52, 53, 54, 55, and 56, as shown in FIG. 10. Thisstructure can prevent the deterioration of the isolation.

Although first ground electrode 4 is not connected to second groundelectrode 13 in the above embodiment, the former and the latter may beconnected to each other through connection via holes 57, 58, 59, and 60as shown in FIG. 11. The connection between the former and the latter tostrengthen the ground can provide a further increased isolation.

The above embodiment showed an example in which transmission line 11shown in FIG. 5 is connected to first ground electrode 4 throughconnection via holes 29, 30, 31, 32, 33, 34, and 35. However, anotherstructure may be also provided as shown in FIG. 12 in which transmissionline 11 is connected to second ground electrode 13 through connectionvia holes 61, 62, 63, 64, 65, 66, and 67. This structure provides aneffect for further reducing the electric field coupling betweentransmission line 6 and transmission line 10. When the electric fieldcoupling between transmission terminal 1 and antenna terminal 3 of thepackage having the structure as described above is represented as acapacitor, the capacitance is about 0.00076 pF at 2.17 GHz, which isabout 36% lower when compared to the conventional example. This isrepresented as an attenuation by line C of FIG. 13. The attenuation is60.7 dB at 2.17 GHz, which shows a further increase of 1.9 dB.

The above embodiment described a filter having a pass band of 60 MHzfrom 1.92 GHz to 1.98 GHz and an attenuation band of 60 MHz from 2.11GHz to 2.17 GHz between transmission terminal 1 and antenna terminal 3.Also, another filter that has an attenuation band of 60 MHz from 1.92GHz to 1.98 GHz and a pass band of 60 MHz from 2.11 GHz to 2.17 GHz canbe additionally formed between reception terminal 2 and antenna terminal3. In this case, transmission terminal 1, reception terminal 2, andantenna terminal 3 are divided by transmission line 11, thus providingan improvement in the isolation characteristic among the respectiveterminals. In any of the above cases, the pass band has a lowerfrequency than that of the attenuation band.

When a further wider band is required, a pass band or an attenuationband of 60 MHz or more and 1 GHz or less can be provided.

The SAW filter of the present invention provides a superiorcharacteristic in a frequency band from 100 MHz to 5 GHz and isparticularly suitable in the use in a radio frequency band.

INDUSTRIAL APPLICABILITY

The SAW filter of the present invention is useful for providing aso-called duplexer for branching a transmission signal and a receptionsignal used for a radio frequency circuit in a mobile communicationdevice (e.g., cellular phone). The SAW filter of the present inventionis particularly useful for a duplexer used for a system using a CDMAmethod for example that has wide pass band and attenuation band and thatis suitable for recent data communication having a large amount.

1. A surface acoustic wave filter comprising: a first dielectric layerhaving a first ground on its lower face and a plurality of transmissionlines on its upper face; a second dielectric layer provided at the upperface of the first dielectric layer and having a second ground on itsupper face; a surface acoustic wave element provided at the upper faceof the second dielectric layer; and a cavity provided at the upper faceof the second dielectric layer and surrounding the surface acoustic waveelement, wherein: one end of the plurality of transmission lines isconnected to the surface acoustic wave element and the other end thereofis connected to the first or second ground; the surface acoustic wavefilter has an additional transmission line between two neighboringtransmission lines among the plurality of transmission lines; and theadditional transmission line is connected to the first or second groundat an interval equal to or lower than a wavelength of a using frequency.2. A surface acoustic wave filter according to claim 1 wherein thesurface acoustic wave element has at the surface thereof an inputterminal, a connection terminal, at least two serial connectedresonators and at least two parallel connected resonators.
 3. A surfaceacoustic wave filter according to claim 1 further comprising one or moredielectric layers having the transmission line.
 4. A surface acousticwave filter according to claim 1 further comprising a metal plate forcovering the cavity.
 5. A surface acoustic wave filter according toclaim 1 further comprising a connection via hole having therein aconductor, wherein the connection via hole connects the first ground tothe second ground.
 6. A surface acoustic wave filter according to claim5 wherein the connection via hole is connected to the transmission line,and the connection via hole has a diameter smaller than a line width ofthe transmission line to be connected thereto.
 7. A surface acousticwave filter according to claim 1 wherein the additional transmissionline has at least one branch.
 8. A surface acoustic wave filteraccording to claim 1 wherein the additional transmission line has atleast one corner section.
 9. A surface acoustic wave filter according toclaim 8 wherein a corner of the corner section has an angle of 45° or90°.
 10. A surface acoustic wave filter according to claim 8 wherein theadditional transmission line at the corner section is connected via aconnection via hole to the first or second ground.
 11. A surfaceacoustic wave filter according to claim 1 further comprising: atransmission terminal, a reception terminal, and an antenna terminal atthe lower face of the first dielectric layer; one transmission line ofthe transmission lines connected to the transmission terminal; anothertransmission line of the transmission lines connected to the receptionterminal; and still another transmission line of the transmission linesconnected to the antenna terminal, wherein the additional transmissionline is disposed between any two neighboring transmission lines amongthe three transmission lines.
 12. A surface acoustic wave filteraccording to claim 1 wherein at least one of a pass band and anattenuation band is 60 MHz or more.
 13. A surface acoustic wave filteraccording to claim 1 wherein the additional transmission line suppressesan electric field coupling between the two neighboring transmissionlines.
 14. A surface acoustic wave filter comprising: a surface acousticwave element; a transmission terminal; a reception terminal; an antennaterminal; and a dielectric layer, the dielectric layer including: afirst transmission line for connecting the transmission terminal to thesurface acoustic wave element; a second transmission line for connectingthe reception terminal to the surface acoustic wave element; and a thirdtransmission line for connecting the antenna terminal to the surfaceacoustic wave element, wherein: the dielectric layer further includes anadditional transmission line between two neighboring transmission linesamong the first, second, and third transmission lines, and theadditional transmission line is connected to a ground at an intervalequal to or lower than the wavelength of a using frequency to suppressan electric field coupling between the two neighboring transmissionlines.